4D imaging and 4D radiation therapy: a New Era of therapy design and delivery.

Recently developed 4D CT imaging technologies have shown that significant organ motion can occur within radiotherapy fields during treatment. Most often a result of respiration, this motion can cause dose delivery errors that are clinically significant when unmanaged, as demonstrated in many recent investigations. Motion during the regular breathing cycling is important, but day-to-day breathing variations, as may be caused by changes in residual tidal volume, can cause systematic shifts in tumor position. These may cause delivery misalignments because the tumor is not in the same average location at each treatment. Approaches to management of this motion may involve motion-inclusive planning, gating or tracking. 4D CT has been instrumental in most of these approaches. Given the state of treatment planning software, it is not possible to preplan whether a specific patient would benefit from one or another of these methods. Daily imaging (or use of a nonimage-based system such as Calypso) is necessary to locate the tumor, and the location must be correlated with measurements from a system that tracks breathing motion during treatment delivery. This is typically done using an independent metric that characterizes the breathing cycle (e.g. the height of the abdomen). Only then can the treatment plan be accurately implemented. There are many methods to manage tumor motion, though most are challenging to implement and remain poorly supported by vendors. When determining which system to use, an important distinction between competing approaches is whether they are amplitude- or phase-based. Some implementations may use different approaches for different parts of the treatment planning and delivery process, potentially introducing errors in the characterization of breathing motion. While many advances have been achieved and are discussed here, the development of solid, stable and robust processes to effectively manage breathing motion remains a foremost and continuing challenge in radiotherapy.